The steam generating apparatus for producing steam from water by an induction heating system is well known in the art. FIG. 20 of the accompanying drawings illustrate a longitudinal sectional view of the prior art steam generator such as disclosed in the Japanese Laid-open Patent Publication No. 4-51487, published in 1992. Referring to FIG. 20, the steam generator 1 includes an iron core 2 around which an electroconductive wire is would to form an induction coil 3. A steam generating tank 5 having its bottom formed by an iron plate 4 capable of creating a magnetic flux circuit is mounted atop the iron core 2 with the iron plate 4 resting on the iron core 2. The prior art steam generator 1 also includes a fluid supply means comprising a water spraying pipe 6 for spraying water onto the iron plate 4 within the steam generating tank 5 and a water supply pump 7, and a steam discharge means comprising a steam discharge pipe 9 having a needle valve 8 disposed thereon. The induction coil 3 referred to above is electrically connected with a commercial AC power source providing an alternating current power of a utility frequency. In this prior art steam generator 1, the iron plate 4 defining the bottom of the steam generating tank 5 serves as a heating element.
Another prior art heating element for heating water or air is disclosed in, for example, the Japanese Laid-open Patent Publication No. 3-98286, published in 1991, and is shown in FIGS. 21 and 22 of the accompanying drawings. Referring to FIGS. 21 and 22, the heating element comprises a generally cylindrical hollow column 10 of insulating material around which a coil 11 is formed, and a laminated filler 13 accommodated within the hollow of the column 10. The laminated filler 13 is made up of a plurality of generally elongated base members 12 each formed with a number of corrugations 4-1, which base members 12 are laminated together with the corrugations in one base member 12 laid so as to intersect the corrugations in the neighboring base member 12. In this structure, when an alternating current is supplied to the coil 11, eddy currents are produced in the laminated filler 13 to allow the latter to evolve heat. Air or liquid flowing through the column 10 as shown by the arrows is heated in contact with the laminated filler 13 then heated in the manner described above.
According to the prior art steam generator shown in FIG. 20, the heating element used as a bottom of the steam generating tank 5 flat, having its opposite surfaces parallel to each other, and has a relatively small surface area at which heat exchange takes place. Therefore, the amount of heat supplied per unitary surface area, that is, the amount of the fluid medium vaporized, is limited. In order to increase the amount of the fluid medium vaporized, the surface area of the heating element must be increased, resulting in increase of the size of the steam generator as a whole.
Also, the metallic material forming the heat element has a substantial thickness and is bulky in terms of heat capacity, exhibiting a relatively low response to heat. For this reason, the amount of the fluid medium vaporized cannot be controlled accurately.
Moreover, since the heating element is disposed at the bottom of the steam generating tank, not only is the prior art steam generator unable to heat the steam once produced to produce steam of an increased temperature, but also the heating speed at which the steam is heated cannot be controlled.
In the case of the heating element in which the laminated filler is employed, the base members forming the laminated filler are electrically coupled with each other through points of intersection between the corrugations 4-1 and 4-2 in the neighboring base members and, therefore, the laminated filler is susceptible to a localized heating that takes place at the points of intersections of the corrugations under the influence of the induction current. For this reason, the heating element utilizing the laminated filler is difficult to accomplish an efficient induction heating.
In addition, since the heating element is designed to heat only liquid or air, no simultaneous or selective production of steam and hot air is possible although only steam or hot air can be produced.